In one approach to materials processing, a substrate is exposed to excited constituents such as ions or neutral radicals that interact physically or chemically with the substrate so as to effect deposition of material onto or removal of material from the substrate. The excited constituents are part of an excited gaseous processing medium generated by applying electromagnetic excitation to a reactant gas composition. Generally, a given processing operation is optimized by a specific processing medium chemistry, defined by the identities and concentrations of the active constituents. To some extent, the chemistry of the processing medium may be fixed by judicious choice of process parameters such as temperature, pressure, gas composition, gas flow rates, reactor geometry, and applied power. However, the complex dependence of the processing medium's composition on the process parameters limits the extent to which any single compositional variable may be adjusted independently of another one. Often, changes made in one processing parameter in order to adjust the concentration of one constituent adversely alter the identity or concentration of another critical constituent, or change other aspects of the process in an undesirable way. Consequently, the typical processing sequence uses a combination of process parameters which are selected to compromise among, or to optimize a single one of, the various competing process performance characteristics.
Plasma etching techniques, such as are widely used in the semiconductor industry, illustrate how the interaction of the process parameters contains the processing medium chemistry. In many common etching processes, the processing medium contains both ionic and neutral radical elements. Material is removed from the substrate by relatively volatile species created by reaction of radicals with the substrate material; ions impinging the surface may provide the energy needed to eject substrate material from the substrate so it can react with a radical or may volatilize products residing on the substrate.
The plasma is generally generated by applying, an oscillating electromagnetic field to a reactant gas composition in order to excite collisions between the molecules that result in ionization or other excitation. Many specific approaches to applying this excitation have been developed. Parallel plate reactors, as shown in U.S. Pat. Nos. 4,626,312 and 5,248,371, are the most elementary plasma etching systems. The plasma is generated in situ between the plates by a radio frequency ("rf") electrical field oriented perpendicularly to the substrate.
Other reactor designs have been devised to broaden the range of possible process parameters values so as to improve etch characteristics such as rate, anisotropy, or selectivity. Etching systems have commonly incorporated additional electrical or magnetic power sources. For example, Skidmore, Semiconductor International, 1989, pp. 74-79 and U.S. Pat. No. 4,668,338 describe systems incorporating additional magnetic fields for enhancing the plasma density. Some designs have also removed the creation of the reactive constituents from the vicinity of the substrate. In the planar plasma technique described in U.S. Pat. No. 4,948,458, the plasma is generated in the region of the chamber opposite the substrate by a rf current resonated through a planar coil disposed outside of the reactor chamber. In so-called "downstream" processes, the plasma is created upstream of a main etching chamber with the reactive constituents being subsequently transported to the main chamber where the etching takes place. For example, the system described in U.S. Pat. No. 4,368,092, herein incorporated by reference, generates the plasma in a tubular chamber, with an external helical inductive resonator, in fluid communication with a chamber in which the etching occurs. Electron cyclotron resonance ("ECR") techniques, described by Skidmore, generate the plasma in a microwave resonance chamber in communication with an etching chamber.
Plasma etching finds wide application, for example, in fabricating VLSI structures for integrated circuits. The trend towards greater device densities and smaller minimum feature sizes in integrated circuits has imposed increasingly stringent requirements on the basic IC fabrication steps including etching as well as deposition, film formation, and doping. The shrinking scale of VLSI structures has made precise feature delineation more difficult due to the greater criticality of etch anisotropy. Proper rendition of smaller features requires a higher degree of directionality in the transport processes comprising the etch. Collisions between reactant ions or radicals occur less frequently in a rarer medium than in a denser one, thereby allowing for reactant movement along longer straight paths as the plasma pressure is decreased. Lower process pressures also facilitate the elimination of volatile products from the substrate, especially from the interiors of high aspect-ratio features.
This demand for lower processing pressures is problematic for plasma-driven processes because of the critical role the reactant concentration, strongly influenced by the process pressure, plays in determining the etch rate. As the pressure is decreased to sufficiently low levels to satisfy the structure's dimensional requirements, the overall etch process efficiency becomes too low to fabricate devices quickly enough to be practical. The etch rate can be enhanced somewhat by increasing the plasma density, for example by providing more power to the reactor. However, increasing the power level generally requires concomitant adjustment of other process parameters. Furthermore, the efficacy of this approach is limited because at lower pressure, not only is the overall reactant density changed, but also the relative ratios of different types of active constituents; at lower pressures, neutral radical generation is suppressed relative to ionization. Thus, the compositional profile of the processing medium cannot be duplicated at lower pressures simply by increasing the plasma density. And ions in the processing medium, even if present in superabundance, cannot perform the function of a given neutral constituent. Without the contribution of the neutral species, the etch rate is limited.
In these etch chemistries, the opposing effects of an experimental parameter such as pressure on the concentration of different processing medium constituents limits the etch rate attainable at low pressures. Or, conversely, it limits the transport directionality, and thus feature size, achievable at a given etch rate.